1997 T100 auto w/135k miles. Approx 6 years ago started getting a high pitch vibrational metallic noise from what sounds like the drive train. Need to drive for 5-10 minutes for it to start, it’s intermittent, and most common while accelerating at 35-40mph. Noise goes away if I let off the gas or accelerate.

I started hitting the zerks more often, changing fluids, keeping on eye on the magnets, etc. The rear diff had stinky milky fluid with quite a bit of metal chunks on the magnet. I’ve always considered the 3rd member as a source of the noise.

The noise has been slowly getting worse. I recently purchased chassis ears on Amazon. The noise sounds like it’s coming from the 3rd. I dropped the rear driveshaft and drove in 4wd. The sound was still there which has me kind of puzzled.

The noise happens while under load (accelerating). Can it possibly be the 3rd if the noise was still there while driving with no rear drive shaft?

Sorry for the long winded question.